Previously, an oil-cooled screw compressor was composed such that, lube oil supplied through an injection-supply line for the purpose of sealing the clearance between the male and female rotors and the clearance between the casing and the rotors, and lube oil supplied through a supply line for lubricating the bearings join together in the rotor room where sucked gas is being compressed, and the interflowed lube oil is discharged together with the compressed gas. Therefore, lube oil of the same kind have to be supplied to both the supply lines, because lube oil supplied to each line mixes with each other in the rotor room.
When compressing high condensable gas such as high hydrocarbon group gas and water-saturated natural gas of relatively high boiling point, which are highly soluble in lube oil, synthetic lube oil of high viscosity was used as lubricant with its supply amount reduced and its temperature raised. Or the compressor was operated under high temperature by adopting separate supply of lube oil to rotor bearings. The reason of operating under high temperature is to keep the temperature of compressed fluid discharged from the compressor higher than its dew point and to reduce solubility of the compressed fluid in lube oil.
However, operation of the compressor under high temperature causes reduction in strength of bearing material due to heat generation by sliding in rotor bearings or reduction of lifetime of bearings due to lowering in viscosity of lube oil. To solve the problem, it is demanded to develop bearing material of high heat tolerance, but no bearing material superior than white metal has been commercially available as of now as far as economical efficiency and reliability are concerned.
Further, synthetic lube oil has hydrophilic property, and when water or active ingredient was contained in fluid f to be compressed, hydrolysis cleavage or corrosion of bearing material occurred.
In Japanese Laid-Open Patent Application No. 2003-97558 is disclosed material of lubricating part of low friction superior in lubricity and not apt to be seized, in which at least one of two friction surfaces of material bodies contacting one another via lubricant containing a compound having a hydrophilic radical and hydrophobic radical in its molecules, is formed of a micro-phase-separated surface of a compound 1 and the other friction surface is formed of a hydrophilic surface. With the construction of the bearing part, an adsorption film which is hardly torn away is formed on the friction surfaces (particularly on the friction surface of the compound 1), so bearing seizure is not apt to occur. In addition to this, as both the adsorption films are poor in conforming to each other, friction surfaces superior in lubricity can be obtained.
However, in a screw compressor used in a refrigerating system or compression system using as a refrigerant a highly soluble gas which is highly soluble in lube oil, such as high condensable gas having relatively high boiling point, wear resistance of bearings deteriorates rapidly when lube oil temperature exceeds 125° C., and operating life of the compressor is shortened. Abovementioned material of lubricating part does not necessarily provide a lubricating part superior in wear resistance under high temperature. Further, when aluminum alloy or silver is adopted as bearing material, running-in property is not good and bearing seizure is apt to occur.
To solve the problems mentioned above, it may be considered to control temperature and pressure around the bearings taking into consideration the relation between viscosity and temperature depending on dissolution percentage of refrigerant gas dissolved in the lube oil as shown in FIG. 5, and relation between dissolution percentage of refrigerant gas dissolved in lube oil and temperature depending on pressure as shown in FIG. 6. It is recognized from FIG. 5 and FIG. 6 that, the amount of dissolution of refrigerant gas in lube oil decreases when temperature is raised and pressure is decreased, and that viscosity of lube oil lowers with decreased pressure. Therefore, by increasing temperature and decreasing pressure of lube oil supplied, the lube oil is maintained in a high viscosity and necessary thickness of oil film for lubrication can be maintained, so it is expected to prolong the lifetime of the bearings. However, there is a restriction to increasing temperature that, when the temperature of lube oil is raised, occurrence of bearing seizure and reduction of bearing life are induced. As to pressure, there is a restriction that pressure of lube oil injected into the rotor room must be high than a certain pressure, for when supply pressure of lube oil to the balance piston to reduce thrust force exerting on the male rotor from discharge side toward suction side, the thrust load of the thrust bearing increases resulting in reduced life of the thrust bearing.